Radio Frequency IDentification (RFID) systems typically include RFID tags and RFID readers (the latter are also known as RFID reader/writers or RFID interrogators). RFID systems can be used in many ways for locating and identifying objects to which the tags are attached. RFID systems are particularly useful in product-related and service-related industries for tracking large numbers of objects being processed, inventoried, or handled. In such cases, an RFID tag is usually attached to an individual item, or to its package.
In principle, RFID techniques entail using an RFID reader to interrogate one or more RFID tags. The reader transmitting a Radio Frequency (RF) wave performs the interrogation. A tag that senses the interrogating RF wave responds by transmitting back another RF wave. The tag generates the transmitted back RF wave either originally, or by reflecting back a portion of the interrogating RF wave in a process known as backscatter. Backscatter may take place in a number of ways.
The reflected-back RF wave may further encode data stored internally in the tag, such as a number. The response is demodulated and decoded by the reader, which thereby identifies, counts, or otherwise interacts with the associated item. The decoded data can denote a serial number, a price, a date, a destination, other attribute(s), any combination of attributes, and so on.
An RFID tag typically includes an antenna system, a power management section, a radio section, and frequently a logical section, a memory, or both. In earlier RFID tags, the power management section included an energy storage device, such as a battery. RFID tags with an energy storage device are known as active tags. Advances in semiconductor technology have miniaturized the electronics so much that an RFID tag can be powered solely by the RF signal it receives. Such RFID tags do not include an energy storage device, and are called passive tags.
A typical application for an RFID reader is to read the codes of tags brought in its field of view, which is also called inventorying the tags. Since many tags could be brought in front of the reader at the same time, there is a process for inventorying that forces the tags to be addressed individually, called singulation.
Stingulation often uses a technique called the slotted aloha technique. The slotted aloha technique distributes the RFID tag population in sequential slots, which will be described later in more detail. The reader determines a value for a Q-parameter and transmits it to the tags. The tags occupy the slots according to random numbers. When a tag occupies a single slot, it can be read out individually, without interference from the others.
When first encountering a population of RFID tags, the reader does not know how many they are. Yet it chooses, as a guess, a value for the Q-parameter that creates slots. There is an optimum number of slots to create for a population of a given size, and if the reader makes a poor guess, it will not inventory the tags quickly.
Improvements to the slotted aloha technique have been made when the reader further runs an algorithm that can change the value of the Q parameter, depending on what is being encountered in the field. Such an algorithm is called a Q-algorithm. If a better value of Q is reached, then inventorying can be accelerated. Such improvements are described, for example, in copending U.S. patent application Ser. Nos. 11/210/573, 11/210,575, and 11/210,422, all filed on Aug. 24, 2005, all due to be assigned to the same assignee, and all incorporated herein by reference.
A requirement for a Q-algorithm is that it learns how tags are distributed in the slots it has created, as it is examining the slots. A problem with this is interference from noise sources in the environment. These may mask tag responses, or be mistaken by the reader as a true tag response. In addition to giving false readings from tags, they can also mislead the Q-algorithm.